1. Field of the Invention
The present invention relates to contaminant sorting devices and, more particularly, to a method and apparatus for sorting contaminant material such as metal and rocks from processing material such as wood chips and the like.
2. Prior Art
In the wood processing industry as well as in many other similar industries, the material being processed, such as wood chips, is often contaminated by material which cannot be processed, such as tramp metal or rocks. Referring specifically to the wood processing industry to which the present invention is more particularly related, it is necessary to remove this contaminant material at an early stage of the processing so as to prevent damage to machinery or production of a materially poor product.
In the wood processing industry there are many processes which involve the reduction of wood particle size by the use of equipment such as chippers, rechippers, flakers and the like, which use, for example, knives or anvils for reducing particle size of the wood chips. If relatively hard material, such as rocks or tramp metal, is introduced into these devices it can cause severe damage to the equipment and result in substantial processing delay due to the necessity of removing the contaminant material and possibly replacing parts in the devices.
Several means are currently used for removing metal objects from the wood material being processed, which generally incorporate magnetic sensing devices which locate metallic objects. However, such devices are ineffective for detecting rocks and similar other nonferro-metallic hard objects which can cause as severe damage as the ferro-metallic objects. In the past, it has been necessary to make visual inspection of the material being processed in order to remove such contaminant, making the process more expensive and susceptible of human error which often results in damage to the processing equipment due to failure to spot the contaminant material.
More recently, equipment has been developed which is capable of accurately sensing the existence of nonferro-metallic objects, such as rocks, and provides for automatic removal of that portion of the material being processed which includes the contaminant. Such a device is disclosed, for example, in U.S. Pat. No. 4,147,620. This device provides an inclined, flat surface on which the processing material is dropped and which acts as a sounding board for a transducer attached to the back of the surface. The vibration produced by impact of a contaminant object is different from that of the vibration produced by wood chips, which difference is discernable by the sensing device. The sensing device activates a trap door as that portion of processing material containing the contaminant slides down the inclined surface and is then diverted from the main flow of processing material. After the contaminant material has been diverted and the trap door closes, the subsequent processing material will pass along its normal flow path to be collected for use in subsequent processing.